Prior art methods of forming porous rigid structures have mandated the use of particles of a uniform size. One such method is disclosed in U.S. Pat. No. 3,166,615 granted Jan. 19, 1965 in the name of J. A. Farrell. Typically, the use of particles of a uniform size requires careful monitoring of the coating process to ensure that the resin layer is likewise uniform. Variations in the resin layer surrounding the particles may result in the filling of the interstices between the particles on compacting thereby resulting in decreased porosity.
Another disadvantage associated with the prior art includes a lack of durability of the final porous rigid structure, due to the less then optimum mechanical interlock of particles of a uniformed size and spherical in configuration when compressed. Additionally, uniform size particles which do not readily interlock are subject to low tensile strength thus producing an inferior structure subject to stress cracking. Durability of the surface layer is also reduced when particles of the uniform size and spherical in configuration are used. This leads to increased wear caused by friction and greater surface vulnerability to damage during use. Premature warping of the final porous structure in use is another problem associated with the prior art. This problem may be over come by the addition of several substructure layers. However, the addition of many substructure layers substantially increases the cost of producing the porous structure.